This invention relates to telephone communication systems and, more particularly, to an improved electronic private automatic branch exchange (PABX). More particularly still, the invention relates to an improved tone signal injection circuit and method for injecting audio tones for informing calling parties of the progress of a call within a PABX telephone system.
Private automatic branch exchanges traditionally have incorporated all of the switching techniques normally utilized in telephone central offices. Many of these types of private switching systems employ the well-known step-by-step or "Strowger" principle, while still others are of the common control type employing cross bar switches or similar devices as the technique for establishing a path between two stations.
The introduction of electronic techniques and circuitry to the telephone communication field to date has found its greatest utilization in the area of central office switching and signaling transmission. Until recently, the usage of these techniques in PABX telephone systems has been limited primarily because of cost considerations. Certain recent developments primarily in the area of common control equipment and particularly memory circuitry have made the design of electronic PABX's more attractive economically. Use of stored program common control and solid state devices permits a considerable reduction in the amount of equipment installed in customer premises.
In the above-identified related patents and application, the described private automatic branch exchange is electronically implemented and employs common control equipment of a generally conventional type and operation. The system is a two-wire system using junctors as a means of connecting two line circuits together via a solid state crosspoint matrix. The junctor has two ports on the outlet side of the matrix and the lines appear as inlets on the matrix.
The preferred embodiment of the invention disclosed herein is for use in a PABX system of the type disclosed in these related patents and application, and the latter are incorporated herein and made a part hereof as though fully set forth.
In such PABX systems, the calling party is informed by various different audio tones of the progress of a call within the system. For example, a returned dial tone means the equipment is ready to accept dialed digits or TCMF signals, a line busy tone indicates that the called line has been reached, but that it is busy, a reorder tone signifies that all outgoing CO trunks (PABX systems) are busy and the caller should wait, and then retry the call. A tone also is returned if the called telephone is not available. In the case of a dial tone and not available tone, tones of different frequencies are used. Tones for line busy and reorder are identical except their rates of interruptions are different.
Since the junctor circuits in the subject PABX system are areas of transmission path circuit concentration, tone injection at this point would reduce circuit redundancy. If solid state devices, that is, PNPN, SCR, etc. are used as crosspoints for the transmission path, the devices' minimum hold current must be exceeded to assure the transmission path is maintained. Since junctor circuits are transformer coupled to the line circuits, any current or voltage fluctuation at the junctor or in the transmission path is coupled and reappears as audio interference at the line circuit.
If a constant current is generated by a constant current source in the junctor circuit, and is coupled to the line transformer secondary via the solid state crosspoints, current fluctuations can be reduced and it can be assured that the transmission path is maintained.
Since transistors are basically current devices, such a constant current source or generator can be designed using a transistor. For example, if the current through the transistor's emitter is held constant, the output current at its collector circuit is essentially the same regardless of the collector's resistor load (within the applied voltage and current rating of the transistor used). The emitter current, in turn, is controlled by the transistor's base circuit.
In accordance with the present invention, a precise tone injection circuit can be provided, by injecting tone signals at the transistor's base circuit to change the current through the collector via the emitter circuit at a rate and frequency covered by the tone signal at the base. By current modulating the constant current source, the output frequencies are not altered or distorted. To minimize noise pick up, within the transmission path, a balanced two-wire circuit is used to the line transformer. In this fashion, signals of the same amplitude and phase normally created by noise or voltage fluctuation are rejected.
The disclosed precise tone injection circuit therefore provides the function to inject tone signals to a constant current solid state junctor circuit. Since a two-wire balanced circuit is used, and the injected signal currents to each wire are 180.degree. out of phase, in phase signals, i.e., noise and voltage fluctuations, are reduced or rejected. In addition, this circuit reverts to a constant current generator when tone signal generation is not required.
Accordingly, it is an object of the present invention to provide an improved precise tone injection circuit.
Another object is to provide an improved precise tone injection circuit and method, by current modulation on balanced two-wire solid state crosspoint constant current junctor circuits.